Cell division occurring during cleavage

The correct option is closed mitosis.

Cleavage is the sequence of rapid cell divisions that take place immediately after fertilization, transforming the single-celled zygote into a multicellular structure. During this phase, cell divisions occur without any cell growth, meaning the overall volume of the embryo remains constant while the number of cells increases.

Mitosis is broadly classified based on the behavior of the nuclear envelope during division:
• Open Mitosis: The nuclear envelope breaks down completely during prophase and is reassembled at the end of cell division.
• Closed Mitosis: The nuclear envelope remains fully intact throughout the cell division cycle. The mitotic spindle forms inside the nucleus or penetrates it without leading to nuclear envelope disassembly.

During the early cleavage stages of embryonic development in many organisms, the nuclear divisions are closed (or semi-closed) mitotic divisions. This process allows for extremely rapid, synchronous replication and division cycles without the time-consuming process of breaking down and rebuilding the nuclear envelope in each cycle.

Let us analyze the other options:
• Amitosis: This is a form of direct division without spindle formation or chromosome condensation, typical of unicellular organisms or specialized cells, and is not the method of embryonic development.
• Meiosis: This is a reductional cell division that produces haploid gametes, whereas cleavage requires mitotic proliferation to maintain the diploid chromosome number.
• Mitosis: Although cleavage is mitotic, closed mitosis is the more specific and accurate biological description for the divisions occurring during this phase.